New standards continue to improve bandwidth in wireless networks. Typically the increase in bandwidth comes from improved algorithms that can pack more data into the same frequency range. Standard 802.11g has five times the bandwidth of standard 802.11b on the same frequency while standard 802.11n has five times the bandwidth on the same frequency as standard 802.11a.
However, in each of the above examples the old and new standards use the same frequency in the same space, so a client using the old frequency forces other devices within range to back off or revert to the old standard. A single client at the old standard would force clients at the new standard to interface with the network at the old standard, reducing the bandwidth in the wireless network.